This invention disclosure describes a method for realizing a long chirped planar-waveguide Bragg grating with Archemedes' spiral geometry that can fit in a small area (˜1 m can be realized in an area of 1 cm2) and provide large dispersion. The ability to form the grating in such a small area enables fabrication with only a single photolithography mask. Therefore the large physical translation associated with long CFBG fabrication methods is avoided, and the associated specialized fabrication equipment is replaced by standard photolithography equipment. Utilizing photolithography fabrication techniques enables fabrication of planar-waveguide Bragg gratings with custom dispersion profiles for different applications.
Two common dispersive devices are Chirped Fiber Bragg Gratings (CFBG) and Dispersion Compensation Fiber (DCF). The formation of gratings in optical fiber is commonly done by UV exposure to the core of the fiber to produce a periodic index modulation. A conventional way to fabricate long CFBG is by stitching together smaller grating segments. A small length of fiber grating is written with a UV interferogram, a translation stage moves the fiber and the next segment is written. This process continues until the whole grating is formed. In order to properly align and stitch together the segments, the precise location of the fiber must be known. The accuracy of the fiber location is limited by the motion stage encoder moving the fiber. Extremely precise tolerances in the mechanical and optical systems must be maintained in order to properly form the complete grating. This has led to the development of specialized precision equipment for fabrication of long gratings. Current CFBG technology can deliver 10 m long 2.5 ns/nm group velocity dispersion over entire optical C band (40 nm) with <4 dB insertion loss.
DCF has dispersion of −100 ps/nm/km with insertion loss of 0.6 dB/km. In order to achieve a large dispersion, a long length of DCF is required. Having a long length of DCF in a system not only increases the overall system size, but also introduces extra insertion loss. For example, 25 km of DCF fiber is required to achieve 2.5 ns/nm dispersion, and introduces 15 dB insertion loss.
This concept is motivated by recent advances in the development of low optical propagation loss planar-waveguides. Integrated photonic device fabricated in silica planar-waveguides has been demonstrated with <2 dB/m propagation loss, where the waveguide material platform design was 15 μm silica under cladding, 4 μm silica core with 1.5% index contrast, 22 μm silica top cladding, on a silicon substrate. Extra-low-loss planar-waveguides has been demonstrated (<0.1 dB/m), where the waveguides are fabricated on a silicon substrate with 15 μm of silica lower cladding, Si3N4 core, and 15 μm silica top cladding.